Which vibration level would not be apparent to experienced aircrew unless attention is directed to them?

• A. Moderate
• B. Slight
• C. Heavy
• D. Severe

Answer: (B)

Perception of vibration depends on the crew’s normal perceptual baseline and where their attention is directed. Experienced aircrew learn to filter out routine cockpit hum and rotor/engine vibrations as part of normal flight. Slight vibrations sit at or just below that baseline, so they typically aren’t noticed unless someone actively focuses on them or instrumentation highlights them. Moderate, heavy, and severe vibrations, by contrast, exceed the usual noise and are readily detected—through feel, checklist cues, or instrument readings—without needing special attention. So the level that would not be apparent unless attention is directed to it is the slight vibration.